1. Field of the Invention
This invention relates to molds used to cast plastic progressive eyeglass lenses, and to methods of making such molds. This invention also relates to lenses cast using these molds and methods.
2. Description of Related Art
Progressive eyeglass lenses typically include a front face, a back face, a near vision correction zone, a far vision correction zone, and a vision transition zone located between the near vision correction zone and the far vision correction zone. The front faces of these lenses are typically substantially convex, and back face of these lenses are typically substantially concave. These lenses are called "progressive" lenses because their power changes gradually and progressively (i.e., from the power of the near vision correction zone to the power of the far vision correction zone) in the vision transition zone. These lenses are generally preferred compared to ordinary or blended bifocal lenses because wearers of progressive lenses can focus on objects at near distances (by looking through the near vision correction zone), far distances (by looking through the far vision correction zone) and distances between near and far distances (by looking through different parts of the vision transition zone). Bifocal lenses only allow wearers to focus on objects at near and far distances since bifocal lenses only have a near vision correction zone and a far vision correction zone.
Molds used to cast progressive eyeglass lenses are hereinafter referred to as "progressive molds." These molds typically include a back face which is adapted to cast: (1) a near vision correction zone in the lower portion of a lens when worn, (2) a far vision correction zone in the upper portion of a lens when worn, and (3) a vision transition zone between the near and the far vision correction zones. Progressive molds are typically used to cure lens forming compositions to form lenses. They are typically used in combination with a second "back" companion mold and a gasket to form a mold cavity. In one embodiment the progressive mold and companion mold seal against opposite sides of the gasket to form the mold cavity. The mold cavity is then filled with a curable liquid composition, and the liquid composition is cured to a solid by exposure of the composition to light or heat.
Plastic eyeglass lenses are typically cured in the form of a lens "blank" which is close to, but generally not identical to, the prescription needed by the lens wearer. This blank must be further shaped (e.g. cut, beveled, or ground) to closely fit specific customer prescriptions. This final "prescription shaping" may be completed in a retail environment, or a wholesale laboratory environment.
Recently new methods have been developed for preparing lenses by casting lens-forming compositions between glass molds such that the cast lenses fit specific wearer prescriptions. These cast lenses do not require "prescription shaping" since they are cast to meet the final prescription needs of the wearer. The lens-forming compositions in these processes may be cured by exposure to ultraviolet light or heat, or by any free radical polymerization process. Ultraviolet light processes are adaptable to produce lenses in time periods of less than one hour. As a result, lenses can be directly cast, without further shaping, to fit customer prescriptions and still provide quick service. Some of such curing techniques are described in the U.S. patents and co-pending applications described below:
co-pending U.S. application entitled "Apparatus and Process For Lens Curing and Coating" by Buazza et al., U.S. application Ser. No. 931,946, filed Aug. 18, 1992, now U.S. Pat. No. 5,529,728 which is a continuation-in-part of co-pending U.S. application serial no. 800,561, filed Dec. 6, 1991, now U.S. Pat. No. 5,415,816 which is a continuation-in-part of co-pending U.S. application Ser. No. 642,614, filed Jan. 17, 1991, abandoned in favor of U.S. application Ser. No. 180,836, filed Jan. 13, 1994, now U.S. Pat. No. 5,364,256 which is a continuation-in-part of co-pending U.S. application Ser. No. 425,371, filed Oct. 26, 1989, which is a continuation-in-part of Ser. No. 273,428, filed Nov. 18, 1988, now U.S. Pat. No. 4,879,318, which is a continuation-in-part of Ser. No. 021,913, filed Mar. 4, 1987, now abandoned, which is a continuation-in-part of Ser. No. 823,339, filed Jan. 28, 1986, now U.S. Pat. No. 4,728,469.
These patents and co-pending applications are incorporated by reference. In addition, U.S. Pat. No. 4,919,850 also describes methods and apparatus for making plastic lenses, and this patent is also incorporated by reference.
Progressive molds may be made using a variety of methods well known in the art such as grinding. Grinding progressive molds, however, can be unduly expensive and difficult due to the uneven back face of the mold. An alternate method to make progressive molds is a "heat slumping" process. This process is preferred by some manufacturers for preparing progressive molds because it is relatively less expensive than other processes such as grinding.
When making progressive molds using a heat slumping process, a first or prototype surface may be prepared which is a replica of the front face of the progressive lens desired. This first surface may be concave or convex, and typically comprises a ceramic material. After the first surface has been prepared, mold material is placed over this first surface. Typically this mold material is made of glass and is shaped in the form of a disk. The mold material and the first surface are then heated to a high enough temperature such that the mold material "slumps" or molds around the first surface, thereby forming a progressive mold with a back face substantially similar to the front face of the lens desired. The resulting mold also has a front face, and side surfaces connecting the front face to the back face. The mold is then removed from the first surface.
Whether made by grinding, heat slumping, or other processes, one problem with nearly all existing progressive molds is that they tend to cast lenses that have "vertical prism" effects. If uncorrected, vertical prism effects cause wearers of such lenses to see images at different vertical levels than the images really are (i.e., the lenses tend to cause the wearer's vision to shift slightly in the vertical direction). Wearers of lenses with pronounced vertical prism imbalance tend to experience diplopia (double vision), nausea, and/or suffer headaches. Vertical prism effects are believed to be caused by excessive differential thicknesses between the near and far vision correction zones in progressive lenses. Specifically, the bottom of the lens proximate the near vision zone is thicker than the top of the lens proximate the far vision correction zone. If the amount of thickness difference is excessive, it may cause an unacceptably large amount of (base down) vertical prism effects.
Vertical prism effects exist but are relatively easily corrected in progressive lens blanks. The correction takes place when the lens blanks undergo prescription shaping. The prescription shaping process usually involves applying a shaping instrument (such as a grinder) to the center of the concave surface of the lens. To hold the lens blank in place, the convex surface of the lens blank is attached to a lens "block." This lens block is attached to the geometric center of the convex surface of the lens blank. Any desired prism effect correction is made by inserting a "prism ring" around the block such that it can seat against the lens blank. The prism ring is designed to tilt the lens blank the appropriate amount necessary to increase or decrease vertical prism effects in the lens blank when the lens blank is shaped to prescription. The tilting caused by the prism ring also affects the thickness of the edges of the resulting lens. Sometimes practitioners will vary the vertical prism effects (i.e. by varying the prism ring tilt) in an effort to cosmetically optimize lens edges. This process is well known in the art.
Vertical prism effects are more difficult to treat with lenses that cast to prescription since these lenses do not undergo a prescription shaping process. Thus vertical prism effects remain in these lenses unless other measures are taken to reduce or eliminate them. These measures are the subject of the invention described herein.